The present invention relates to an image-pickup apparatus such as a video camera, and more particularly, to an image-pickup apparatus that performs so-called hybrid AF in which TV-AF and external or internal phase difference AF are used in combination.
For auto-focus (AF) control of video cameras or other image-pickup apparatuses, a TV-AF method is mainly used in which a video signal is produced by using an image-pickup element, an AF evaluation value signal is produced to represent the sharpness (contrast state) of the video signal, and the position of a focus lens where the AF evaluation value signal is at the highest is searched for.
AF methods include an external ranging method or external phase difference detection method in which a ranging sensor is provided independently of an image-pickup lens to detect a distance to an object, the in-focus position of a focus lens is calculated on the basis of the distance, and the focus lens is moved to the in-focus position.
In the external phase difference detection method, light flux from an object is split into two that are each received by a pair of light-receiving element arrays (line sensors). A difference between images formed on the pair of line sensors, that is, a phase difference, is detected and used to determine an object distance with a triangulation method. A focus lens is moved to a position where the lens is focused for that object distance.
The AF methods also include an internal phase difference detection method in which light flux passes through an exit pupil of the image-pickup lens and is then split into two which are each received by a pair of line sensors. A phase difference between two images on the pair of line sensors is detected, a defocus amount of the image-pickup lens is determined from the phase difference, and a focus lens is moved by an amount corresponding to the defocus amount.
Japanese Patent Laid-Open No. 5(1993)-64056 has proposed a hybrid AF method realized by combining those AF methods in order to take advantage of high accuracy of focusing in the TV-AF method and quick focusing in the phase difference detection method. The hybrid AF method proposed in Japanese Patent Laid-Open No. 5(1993)-64056 involves moving a focus lens to near an in-focus position by the phase difference detection method and then moving the focus lens to the in-focus position more accurately by the subsequent TV-AF method.
In the conventional hybrid AF, a focus lens is first moved to an in-focus position obtained with the phase difference detection method, and only when the AF evaluation value at that point is high, the TV-AF method follows to move the focus lens to an in-focus position obtained with the TV-AF method.
In moving the focus lens to the in-focus position in the phase difference detection method generally having lower focusing accuracy than the TV-AF method, however, the focus lens may be moved beyond (overshoot) the in-focus position obtained with the TV-AF method. Also, the movement of the focus lens may be stopped temporarily before the in-focus position obtained with the TV-AF method.
In the former case, the focus state of displayed video noticeably varies to reduce image quality. In the latter case, the temporary stop of the focus lens may increase the time taken to achieve focus in the TV-AF method, or may cause an unsmooth change in the focus state of displayed video until an in-focus state is obtained in the TV-AF method, thereby making a user feel anomalous.
In addition, especially when the external phase difference detection method is used, the view field of the ranging sensor is shifted from the view field (image-pickup area) in the TV-AF method for a certain object distance, that is, parallax is present. Thus, The external phase difference detection method and the TV-AF method may calculate the in-focus positions of the focus lens for objects different from each other. In such a case, the focus lens is moved to an incorrect position that is calculated as the in-focus position in the phase difference detection method, so that the resulting video is significantly out of focus.